From what I gather, Google deliberately uses silo management (or whatever it is called). An engineer's performance reviews are based almost entirely on tangible work (lines of code) on his assigned product. Work on intangibles (mentoring, marketing strategy), or on tangibles outside the project (cross-product integration, future proofing) count against him.

Ex-Googler Piaw Na has an interesting post about this: http://piaw.blogspot.com/2010/08/tips-for-noogler-engineers....